1. Field of the Invention
The present invention relates to an overrunning clutch wherein rollers are circumferentially disposed in two rows which are radially spaced apart from each other.
2. Description of the Prior Art
FIGS. 6 and 7 are sectional views of a typical conventional overrunning clutch for use, for example, in a starting motor. In these figures, the reference numeral 1 denotes an output rotary shaft which is rotatably coupled to the armature of a d.c. motor (not shown). The shaft 1 has a helical spline 1a formed in the outer periphery therof. A clutch outer member 2 of the overrunning clutch is fitted on the shaft 1 through the helical spline 1a. The clutch outer member 2 has tapered cut portions 2a formed in the inner peripheral portion thereof, the cut portions 2a being spaced equally in the circumferential direction, thereby defining wedge-shaped spaces 4 between the clutch outer member 2 and a clutch inner member 3 which is disposed therein. Columnar rollers 5 are movably disposed within the wedge-shaped spaces 4, respectively. Each roller 5 is pressed toward the narrower side of the corresponding space 4 by means of a spring 6. The clutch inner member 3 has a pinion 7 formed integral with it and is supported through a metal sleeve 8 so that it is rotatable as well as slidable relative to the output rotary shaft 1. The reference numeral 9 denotes a plate which covers one side of the overrunning clutch, 11 a cover which is provided over the plate 9 so as to cover the outer side of the overrunning clutch through a packing 10, and 12 a holder which constitutes an engagement member which is engaged with a shift lever (not shown).
The operation of the above-described prior art will next be explained. As the d.c. motor is driven, the output rotary shaft 1 rotates, thus causing the clutch outer member 2 to be driven in the direction of the arrow. As a result, each roller 5 rotates about its own axis and cuts into the narrower portion of the corresponding wedge-shaped space 4, causing a wedge action. Thus, the rollers 5 engage the clutch outer member 2 and the clutch inner member 3 by means of frictional force. Accordingly, the clutch outer member 2, the rollers 5 and the clutch inner member 3 rotate together as one unit and the rotation of the d.c. motor is thereby transmitted to the pinion 7, causing the ring gear of the engine which is meshed with the pinion 7 to be driven, and thus starting the engine. After the engine is started, the pinion 7 is conversely driven by the engine, and when the rotational speed of the clutch inner member 3 exceeds that of the clutch outer member 2, each roller 5 rotates about its own axis and moves toward the wider side of the wedge-shaped space 4. As a result, the clutch outer member 2 and the clutch inner member 3 are disengaged from each other, thereby preventing transmission of the driving force in the reverse direction.
When an impact torque which is greater than the torque generated by the starting motor is generated in the overrunning clutch, for example, when the starting motor is restarted while the starting motor and the engine are rotating by inertia and the pinion 7 is re-engaged with the ring gear of the engine, each roller 5 abuts against the narrower-side wall of the wedge-shaped space 4, thereby allowing the clutch outer member 2 to race relative to the clutch inner member 3 with a torque higher than a predetermined transmitted torque, and thus absorbing the impact torque.
The conventional overrunning clutch suffers, however, from the following problems. Specifically, the angle of torsion with respect to the torque transmitted from the clutch outer member 2 to the clutch inner member 3, that is, the amount of change in the rotational angle difference between the clutch outer member 2 and the clutch inner member 3 when torque is transmitted from the former to the latter, is small. Accordingly, the amount of energy absorbed by torsion is small, so that when the transmitted torque is large, the constituent elements may be damaged or the function of the overrunning clutch may deteriorate. It may be considered to increase the amount of energy absorbed by increasing the angle of torsion between the clutch outer member 2 and the clutch inner member 3. In such a case, however, the upper limit value of transmitted torque, that is, the value of sliding torque, is lowered and therefore it has heretofore been difficult to increase the amount of energy absorbed without lowering the value of sliding torque.
When the above-described re-engagement of the pinion 7 with the ring gear is carried out relatively frequently, even if the impact torque generated is lower than a predetermined transmitted torque with which the clutch outer member 2 and the clutch inner member 3 race relative to each other, the stress that is generated by the rollers 5 cutting into the respective wedge-shaped spaces 4 may exceed the limit of the elastic deformation of the elements constituting the wedge-shaped spaces 4. If this is repeated, the rollers 5 and the elements constituting the wedge-shaped spaces 4 will be deformed or become worn, resulting in a lowering in the transmitting function of the overrunning clutch and a change in the upper limit value of transmitted torque. Thus, the prior art is inferior in its durability.